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Versions: 00 01 draft-ietf-netmod-yang-metadata

NETMOD Working Group                                           L. Lhotka
Internet-Draft                                                    CZ.NIC
Intended status: Standards Track                      September 11, 2014
Expires: March 15, 2015


                 Defining and Using Metadata with YANG
                  draft-lhotka-netmod-yang-metadata-00

Abstract

   This document defines a YANG extension statement that allows for
   defining metadata annotions in YANG modules.  The document also
   specifies the encoding of annotations and rules for annotating
   instances of YANG data nodes.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 15, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Defining Annotations in YANG  . . . . . . . . . . . . . . . .   5
   4.  The Encoding of Annotations . . . . . . . . . . . . . . . . .   6
     4.1.  XML Encoding  . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  JSON Encoding . . . . . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Metadata Object and Annotations . . . . . . . . . . .   7
       4.2.2.  Adding Annotations to Container, Anyxml and List
               Instances . . . . . . . . . . . . . . . . . . . . . .   8
       4.2.3.  Adding Annotations to Leaf Instances  . . . . . . . .   8
       4.2.4.  Adding Annotations to Leaf-list Instances . . . . . .   9
   5.  Representing Annotations in DSDL Schemas  . . . . . . . . . .   9
   6.  Metadata YANG Module  . . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   There is a need to be able to annotate instances of YANG [3] data
   nodes with various metadata.  Typical use cases are:

   o  Deactivating a subtree in a configuration datastore while keeping
      the data in place.

   o  Qualifying the data model information with instance-specific data.
      For example, an annotation may be attached to an instance of a
      leaf with the "union" type to indicate the member type to which
      the instance belongs.

   o  RPC operations may use metadata annotations for different purposes
      in both requests and responses.  For example, the <edit-config>
      operation in the NETCONF protocol (see section 7.2 of [5]) uses
      annotations in the form of XML attributes for identifying the
      point in the configuration and type of the operation.

   However, metadata annotations could potentially lead to
   interoperability problems if they are used in an ad hoc way by
   different organizations and/or without proper documentation.  A sound
   metadata framework for YANG should therefore satisfy these
   requirements:





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   1.  The set of annotations must be extensible in a distributed manner
       so as to allow for defining new annotations without running into
       the risk of collisions with annotations defined and used by
       others.

   2.  Syntax and semantics of annotations must be documented and the
       documentation must be easily accessible.

   3.  Clients of network management protocols such as NETCONF [5] or
       RESTCONF [10] must be able to learn all annotations supported by
       a given server and identify each of them correctly.

   This document proposes a systematic way for defining and using
   metadata annotations that satisfies the above requirements.  For this
   purpose, YANG extension statement "annotation" is defined in the
   module "ietf-yang-metadata" (Section 6).  Other YANG modules
   importing this module can use the "annotation" statement for defining
   one or more annotations.

   The benefits of defining metadata annotations in a YANG module are as
   follows:

   o  Each annotation is bound to a YANG module name, namespace URI and
      prefix.  This makes its encoding in instance documents (both XML
      and JSON) straightforward and consistent with the encoding of YANG
      data node instances.

   o  Annotations are indirectly registered through IANA YANG module
      registration.

   o  Annotations are included in the data model.  Specifically, servers
      indicate support for certain annotations using standard module
      advertisement methods, such as the <hello> message in NETCONF.

   o  Values of annotations need not be strings; any YANG built-in or
      derived type may be used for them.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [1].

   The following terms are defined in [5]:

   o  client,

   o  datastore,



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   o  message,

   o  operation,

   o  server.

   The following terms are defined in [3]:

   o  anyxml,

   o  built-in type,

   o  derived type,

   o  container,

   o  data model,

   o  data node,

   o  derived type,

   o  extension,

   o  leaf-list,

   o  list,

   o  module,

   o  RPC operation,

   o  submodule,

   o  type.

   The following terms are defined in [8]:

   o  attribute,

   o  document,

   o  element,

   o  namespace,

   o  prefix.




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   The following terms are defined in [6]:

   o  array,

   o  member,

   o  object,

   o  primitive type.

   XML element names and YANG extension statements are always written
   with explicit namespace prefixes that are assumed to be bound to URI
   references as shown in Table 1.

        +--------+------------------------------------------------+
        | Prefix | URI Reference                                  |
        +--------+------------------------------------------------+
        | rng    | http://relaxng.org/ns/structure/1.0            |
        | md     | urn:ietf:params:xml:ns:yang:ietf-yang-metadata |
        | ein    | http://example.org/example-inactive            |
        +--------+------------------------------------------------+

     Table 1: Used namespace prefixes and corresponding URI references

3.  Defining Annotations in YANG

   Metadata annotations are defined with YANG extension statement
   "md:annotation".  This YANG language extension is defined in the
   module "ietf-yang-metadata" (Section 6).

   Substatements of "md:annotation" are shown in Table 2.  They are all
   core YANG statements, and the numbers in the second column refer to
   the corresponding sections in RFC 6020 [3] where each statement is
   described.

             +--------------+------------------+-------------+
             | substatement | RFC 6020 section | cardinality |
             +--------------+------------------+-------------+
             | description  | 7.19.3           | 0..1        |
             | reference    | 7.19.4           | 0..1        |
             | status       | 7.19.2           | 0..1        |
             | type         | 7.6.3            | 0..1        |
             | units        | 7.3.3            | 0..1        |
             +--------------+------------------+-------------+

                Table 2: Substatements of "md:annotation".





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   Using the "type" statement, a type may be specified for the
   annotation value according to the same rules as for YANG leaf or
   leaf-list types.  However, the "type" statement is optional as a
   substatement of "md:annotation" statement.  If it is not present, the
   built-in "string" type is the default.

   For example, the following module defines the "inactive" annotation:

   module example-inactive {
     namespace "http://example.org/example-inactive";
     prefix "ein";
     import ietf-yang-metadata {
       prefix "md";
     }
     md:annotation inactive {
       type boolean;
       description
         "If this annotation is attached to a configuration data node,
          and its value is 'true', then the server MUST behave
          as if the data subtree rooted at this node was not
          present.";
     }
   }

   Metadata annotations defined with the "md:annotation" statement may
   be attached to any valid instance of a data node, i.e., container,
   leaf, list, leaf-list or anyxml, throughout the data model.  Metadata
   annotations are always optional.

4.  The Encoding of Annotations

   XML attributes are a natural choice for encoding metadata in XML
   instance documents.  For JSON [6], there is no generally established
   method for encoding metadata.  This document thus introduces a
   special encoding method that is consistent with the JSON encoding of
   YANG data node instances as defined in [7].

4.1.  XML Encoding

   Metadata annotations are added to XML-encoded instances of YANG data
   nodes as XML attributes according to these rules:

   o  The local name of the attribute SHALL be the same as the name of
      the annotation specified in the argument of the corresponding
      "md:annotation" statement.

   o  The namespace of the attribute SHALL be identified by the URI that
      appears as the argument of the "namespace" statement in the YANG



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      module where the annotation is defined.  It is RECOMMENDED that
      the prefix specified by the "prefix" statement in the same module
      is used in the qualified name of the attribute.

   o  The attribute value SHALL be encoded in the same way as the value
      of a YANG leaf instance having the same type.

   For example, the "inactive" annotation as defined in Section 3 may be
   encoded as follows:

       <foo xmlns:ein="http://example.org/example-inactive"
            ein:inactive="true">
         ...
       </foo>

4.2.  JSON Encoding

   The metadata encoding defined in this section has the following
   properties:

   1.  The encoding of YANG data node instances as defined in [7] does
       not change.

   2.  Namespaces of metadata annotations are encoded in the same way as
       namespaces of YANG data node instances, see [7].

4.2.1.  Metadata Object and Annotations

   All metadata annotations assigned to a YANG data node instance are
   encoded as members (name/value pairs) of a single JSON object,
   henceforth denoted as the metadata object.  The placement and name of
   this object depends on the type of the data node as specified in the
   following subsections.

   The name of a metadata annotation (member of the metadata object)
   SHALL be of the following form:

       MODULE_NAME:LOCAL_NAME

   where MODULE_NAME is the name of the YANG module in which the
   annotation is defined, and LOCAL_NAME is the name of the annotation
   specified in the argument of the corresponding "md:annotation"
   statement.

   Note that unlike YANG data node instances, for annotations the
   explicit namespace identifier (MODULE_NAME) must always be used.





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   The value of a metadata annotation SHALL be encoded in exactly the
   same way as the value of a YANG leaf node having the same type as the
   annotation.

4.2.2.  Adding Annotations to Container, Anyxml and List Instances

   For an instance that is translated to a JSON object (i.e., a
   container, anyxml or list entry), the metadata object is added as a
   new member of that object with the name "@".

   Examples:

   o  "cask" is a container or anyxml node:

       "cask": {
         "@": {
           "example-inactive:inactive": true
         },
         ...
       }

   o  "seq" is a list whose key is "name", annotation "inactive" is
      added only to the first entry:

       "seq": [
         {
           "@": {
             "example-inactive:inactive": true
           },
           "name": "one",
           ...
         },
         {
           "name": "two",
           ...
         }
       ]

4.2.3.  Adding Annotations to Leaf Instances

   For a leaf instance, the metadata object is added as a sibling name/
   value pair whose the name is the symbol "@" concatenated with the
   identifier of the leaf.

   For example, if "flag" is a leaf node:






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       "flag": true,
       "@flag": {
         "example-inactive:inactive": true
       }

4.2.4.  Adding Annotations to Leaf-list Instances

   For a leaf-list instance, which is represented as a JSON array with
   values of a primitive type, annotations may be assigned to one or
   more entries by adding a name/array pair as a sibling the leaf-list
   instance, where the name is the symbol "@" concatenated with the
   identifier of the leaf-list, and the value is a JSON array whose i-th
   element is the metadata object with annotations assigned to the i-th
   entry of the leaf-list instance, or null if the i-th entry has no
   annotations.

   Trailing null values in the array, i.e., those following the last
   non-null metadata object, MAY be omitted.

   For example, in the following leaf-list instance with four entries,
   the "inactive" annotation is added to the second and third entry in
   the following way:

       "folio": [6, 3, 7, 8],
       "@folio": [
         null,
         {"example-inactive:inactive": true},
         {"example-inactive:inactive": true}
       ]

5.  Representing Annotations in DSDL Schemas

   RFC 6110 [4] defines a standard mapping of YANG data models to
   Document Schema Definition Languages (DSDL) [9].  This section
   specifies the mapping for the extension statement "md:annotation"
   (Section 6), which enables validation of XML instance documents
   containing metadata annotations.

   The first step of the DSDL mapping procedure, i.e., the
   transformation of the YANG data model to the hybrid schema (see
   sec. 6 in [4]), is modified as follows:

   1.  If the data model contains at least one "md:annotation"
       statement, then a RELAX NG named pattern definition MUST be added
       a child of the root <rng:grammar> element in the hybrid schema.
       It is RECOMMENDED to use the name "__yang_metadata__" for this
       named pattern.




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   2.  A reference to the named pattern described in item 1 MUST be
       included as a child of every <rng:element> pattern that
       corresponds to a container, leaf, list or leaf-list data node.

   3.  Every metadata annotation definition in the form

       md:annotation ARGUMENT;

       or

       md:annotation ARGUMENT {
         ...
       }

       is mapped to the following RELAX NG pattern:

       <rng:attribute name="PREFIX:ARGUMENT">
         ...
       </rng:attribute>

       where PREFIX is the namespace prefix bound to the namespace URI
       of the YANG module that contains the "md:annotation" statement.
       The "rng:attribute" pattern SHALL be inserted as a child of the
       named pattern definition described in item 1.

   4.  Substatements of "md:annotation", if there are any, SHALL be
       mapped to children of the "rng:attribute" pattern exactly as
       described in sec. 10 of [4].

   For example, the named pattern definition (item 1), when constructed
   only for the "inactive" annotation, will have the following form:

       <rng:define name="__yang_metadata__">
         <rng:attribute name="ein:inactive">
           <rng:choice>
             <rng:value>true</rng:value>
             <rng:value>false</rng:value>
           </rng:choice>
         </rng:attribute>
       </rng:define>

   Every "rng:element" pattern that corresponds to a container, leaf,
   list or leaf-list data node will then contain a reference to the
   above named pattern, for example







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       <rng:element name="foo:bar">
         <rng:ref name="__yang_metadata__"/>
         ...
       </rng:element>

   Note that it is not necessary to use such a reference for
   "rng:element" patterns corresponding to anyxml data nodes because
   they already permit any XML attributes to be attached to their
   instances.

   The second step of the DSDL mapping procedure, i.e., the
   transformation of the hybrid schema to RELAX NG, Schematron and DSRL
   schemas, is unaffected by the inclusion of "md:annotation".

6.  Metadata YANG Module

   RFC Ed.: In this section, replace all occurrences of 'XXXX' with the
   actual RFC number and all occurrences of the revision date below with
   the date of RFC publication (and remove this note).

   <CODE BEGINS> file "yang-metadata@2014-09-11.yang"

   module ietf-yang-metadata {

     namespace "urn:ietf:params:xml:ns:yang:ietf-yang-metadata";

     prefix "md";

     organization
       "IETF NETMOD (NETCONF Data Modeling Language) Working Group";

     contact
       "Editor:   Ladislav Lhotka
                  <mailto:lhotka@nic.cz>";

     description
       "This YANG module defines an extension statement that allows for
        defining metadata annotations.";

     revision 2014-09-11 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: Defining and Using Metadata with YANG";
     }

     extension annotation {
       argument name;



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       description
         "This extension allows for defining metadata annotations in
          YANG modules. The 'md:annotation' statement can appear only at
          the top level of a YANG module.

          An annotation defined with this extension statement inherits
          the namespace and other context from the YANG module in which
          it is defined.

          Other properties of the annotation and documentation may be
          specified using the following standard YANG substatements (all
          are optional and may appear only once): 'type', 'description',
          'reference', 'status' and 'units'. If the 'type' statement is
          not present, the built-in 'string' type is used by default.

          A server announces support for a particular annotation by
          including the module in which the annotation is defined among
          the advertised YANG modules (e.g. in NETCONF hello message).
          Depending on the prescribed usage patterns, the annotation
          then may be attached by the server and/or client to any valid
          instance of a data node defined by the server's data model.

          XML and JSON encoding of annotations is defined in
          RFC XXXX.";
     }
   }

   <CODE ENDS>

7.  IANA Considerations

   RFC Ed.: In this section, replace all occurrences of 'XXXX' with the
   actual RFC number (and remove this note).

   This document registers the following namespace URI in the IETF XML
   registry [2]:

   ----------------------------------------------------------
   URI: urn:ietf:params:xml:ns:yang:ietf-yang-metadata

   Registrant Contact: The IESG.

   XML: N/A, the requested URI is an XML namespace.
   ----------------------------------------------------------

   This document registers the following YANG module in the YANG Module
   Names registry [3]:




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   -------------------------------------------------------------------
   name:         ietf-yang-metadata
   namespace:    urn:ietf:params:xml:ns:yang:ietf-yang-metadata
   prefix:       md
   reference:    RFC XXXX
   -------------------------------------------------------------------

8.  Security Considerations

   This document introduces a mechanism for defining metadata
   annotations in YANG modules and using them with instances of YANG
   data nodes.  By itself, this mechanism represents no security threat.
   Security implications of a particular annotation defined using this
   mechanism have to be duly considered and documented.

9.  References

9.1.  Normative References

   [1]        Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [2]        Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [3]        Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2010.

   [4]        Lhotka, L., "Mapping YANG to Document Schema Definition
              Languages and Validating NETCONF Content", RFC 6110,
              February 2011.

   [5]        Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "Network Configuration Protocol (NETCONF)", RFC
              6241, June 2011.

   [6]        Bray, T., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, March 2014.

   [7]        Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              draft-ietf-netmod-yang-json-00 (work in progress), April
              2014.

   [8]        Cowan, J. and R. Tobin, "XML Information Set (Second
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-infoset-20040204, February 2004,
              <http://www.w3.org/TR/2004/REC-xml-infoset-20040204>.



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9.2.  Informative References

   [9]        International Organization for Standardization, "Document
              Schema Definition Languages (DSDL) - Part 1: Overview",
              ISO/IEC 19757-1, November 2004.

   [10]       Bierman, A., Bjorklund, M., Watsen, K., and R. Fernando,
              "RESTCONF Protocol", draft-ietf-netconf-restconf-01 (work
              in progress), July 2014.

Author's Address

   Ladislav Lhotka
   CZ.NIC

   Email: lhotka@nic.cz



































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